Focussing of SAR (Synthetic Aperture Radar) images is generally made, at first, in the radial direction (pulse compression) by analog techniques and later in the azimuth direction using digital techniques or using optical techniques. Digital techniques are preferable because of their flexibility.
Focussing consists in the datum correlation with system response to a point scatterer. This is approximately made up of a linearly frequency modulated sinus wave.
It is to be appreciated that the frequency modulation isn't perfectly linear, and that the pulse response is not localized at constant reflection time, due to the radial migration effect (D. A. Ausharman, A Kosmo, H. M. Jones, E. Poggio. "Development in Radar Imaging" IEE Trans. on Aerospace and Electr. Systems AES-20, n. 4, July 1984). Image focussing requires therefore the convolution of bidimensional matrix data (azimuth and range) using a bidimensional filter, the pulse response of which may extend, in the case of satellite sensors, to up to a thousand samples.
A technique which can be used is that of bidimensional numeric convolution by means of a filter having finite length pulse response, possibly implemented using discrete Fast Fourier Transforms. The drawback is that this technique is not at all flexible and it does not lend itself either to echo radial migration correction, or to system parameter variation due to sensor motion irregularities.
The foremost technique known today uses the step transform with respect over which the technique of this invention is a net improvement (M. Dack, M. R. Ito, I. G. Cunning. "Application of Efficient linear F.M. Matched Fellenng Algorithmsto Synthetic Aperture Radar Processing" IEE Proc. Vol. 13297, No. 1, February 1985).
The step transform technique consists in the conversion of the incoming signal, radially compressed into a frequency chirp having limited time duration T, repeated in time.
If the slope of such chirp is equal but opposite to that of the incoming signal, this is transformed by a constant frequency chirp, into a sequence of sinusoidal signals at piecemeal constant frequency (staircase).
This signal is examined spectrally using techniques which are based upon discrete Fourier transforms.
The amplitude and frequency progression lead to the location of the single scatterer and to an estimate of its reflection coefficient.
This technique, in a rather simple manner, as a first approximation, takes into account both radial migration and small linearity deviations of the frequency modulation law (K. H. Wu, M. R. Vant-"Extension to the Step Transform SAR Processing Technique" IEE Trans. Aerospace an Aerospace and Electr. System, Vol. AES-21 No. 3, May 1985). At any rate, the main drawback which affects this technique is its block structure, which impedes easy adaption.
Furthermore, precise linear F.M. deviation correction isn't possible without reducing the processing advantages which would be obtained otherwise. The technique of radar echo splitting into band limited signals with adjacent bands is known, but is rarely adopted due to its high computing costs when implemented using traditional techniques.
Polyphase networks for signal splitting are known, but have never been applied to synthetic aperture radars.